I think that the big difference today is that they can analyze all data that they can grab. Back in the Good Old Days, you had to devote considerable manpower to this analysis; if you wanted to read letters, you had to be specific because you simply didn't have the resources to look at everyone's mail. This is why most people (including me) have no problem with the idea of monitoring POTS lines; it ties up a considerable amount of resources, so the police are much more likely only to tap the phones they think will get results.

Now, we've gotten to the point where the challenges facing mass surveillance are political rather than physical. I think that this is a lot more dangerous than 1800s police looking at telegraph printouts line by line.